Exogenous 2,4-epibrassinolide and transcriptome profiling reveal the role of brassinosteroid in turnip hypocotyl swelling

Turnip (Brassica campestris L. ssp. rapifera Matzg syn. B. rapa L.) is a biennial plant of the genus Brassica in the family Brassicaceae, agronomically characterized by a unique swollen taproot which develops predominantly from the hypocotyl. However, the regulatory mechanism governing taproot swelling remains largely unexplored. Here, by conducting an integrated morphological, anatomical, and transcriptomic analysis on turnip hypocotyls across vital developmental stages and under exogenous 2,4-epibrassinolide (EBR) treatments, we revealed the important role of brassinosteroid (BR) in promoting hypocotyl swelling. BR orchestrates key regulatory pathways involving plant hormone crosstalk, particularly auxin and cytokinin (CTK), cambium activity, cell wall remodeling, and cell cycle progression during the most critical swelling phase. Many candidate genes associated with these processes were identified for further study. These efforts not only advance our fundamental understanding of BR-driven hypocotyl swelling but also offer potential strategies toward the genetic improvement of turnip and other crops with swollen taproots.